Field of the Invention
The invention relates to a turbine blade, in particular a gas-turbine blade, which extends along a main axis from a root region over a blade body region to a tip region. The invention also relates to a method of producing a turbine blade, in particular a gas-turbine blade.
An apparatus and a method of producing castings, in particular gas-turbine blades, having a directionally solidified structure is described in German Patent DE 22 42 111 B. The method and the apparatus serve to produce castings that as far as possible are free from shrink holes. The directional solidification with a single-crystal or columnar structure is achieved by controlling the start of grain growth. When the method is being carried out, a shell mold to be filled with molten metal is set down on a chill plate and heated to a temperature which is in particular 150.degree. C. above the temperature of the melting point of the metal to be cast. The molten metal is poured into the shell mould, and the chill plate with the shell mold is dipped in a cooling-liquid bath. The temperature of the cooling liquid is substantially below the melting point of the metal. The chill plate has already been cooled by the coolant before the metal is poured into the shell mold. For the production of the turbine blade, the metal used is a superalloy, for example Mar-M 200. The shell mold is dipped in the cooling-liquid bath at such a speed that the surface of the cooling-liquid bath does not run ahead of the solidus level, so that the heat dissipation from the mushy zone of the solidifying alloy takes place vertically downward, and the liquid-solid interface remains essentially horizontal.
This is intended to ensure the growth of a single crystal and to prevent nucleation of grains at the surface of the shell mold. During the production of the turbine blade as a single crystal, the shell mold is heated to over 1500.degree. C. The cooling liquid used is liquid tin, which has a temperature of about 260.degree. C. The speed at which the shell mold is dipped in the liquid bath is about 3 m/h. Here, the turbine blade is cast as a solid-material blade from a nickel-base or a cobalt-base alloy in a single-crystal form having an overall length of about 10 cm.
A speed-controlled method of directional solidification as well as a casting produced according to this method are specified in Published, European Patent Application EP 0 010 538 A1. For the directional solidification of a casting, the ratio of a temperature gradient G and a solidification speed R is especially important. For eutectic superalloys, the ratio of G to R must exceed a certain characteristic value, so that directional solidification takes place. Here, the directional solidification is mainly used in order to produce, for a gas turbine, a casting that is a columnar grain structure, a single crystal or a unidirectional eutectic. The method of directional solidification is used in the case of superalloys such as U-700, B-1900, Mar-M 200 and IN-100. Test trials for producing a gas-turbine blade for the first stage of an aircraft engine in a single-crystal form were carried out at a high dipping speed with radiation cooling and alternatively with cooling by a liquid metal. The speed was between 7.5 cm/h and 33 cm/h in the case of radiation cooling. The directionally solidified casting was cast as a solid body.
A hollow turbine blade for gaseous propellants for turbine rotors of small diameter and having few blades is described in Published, Non-Prosecuted German Patent Application DE 1 007 565 A, in which the overall cross section of the blade increases from the root up to the tip. The increase in the cavity cross section from the root up to the tip is so great that the material cross section narrows from the root up to the tip. The turbine blade consists of two parts, which are connected to one another by brazing, welding or the like.
Described in U.S. Pat. No. 2,916,258 is a turbine, in particular a gas turbine or a steam turbine, which has blades of the same length disposed on a rotor in rows lying in the circumferential direction. In this case, each blade has a mass distribution that differs from the mass distribution of all the other blades of the same rows lying in the circumferential direction. As a result, a certain vibration system, which is intended to reduce the vibrations between the blades, is produced.
A casting process for a gas-turbine blade is described in U.S. Pat. No. 5,072,771. In this case, the melt, for example of a nickel-chrome superalloy, is poured into a casting mold in a furnace provided with a heating zone. After the melt has been poured into the casting mold, the latter is moved out of the heating zone. The turbine blade cast in this way has a grain structure with a multiplicity of randomly oriented grains. The turbine blade has a blade body region, made as a solid body and having a maximum wall thickness of 2 mm, and a root region of solid material having a markedly larger extent. The method of producing long thin moving blades or guide blades in a gas turbine, for reasons of cost, is preferred to methods of producing directionally solidified turbine blades or turbine blades solidified in a single-crystal form.
U.S. Pat. No. 3,465,812 likewise describes the casting of turbine blades having a solid profile.
A method of producing a hollow body which is cast in one piece, can be subjected to a high temperature and has a thin wall is specified in Published, European Patent Application EP 0 750 956 A2. A corresponding casting mold for such a hollow body consists of a ceramic core, which is surrounded by wax and in which a thin silicate layer is applied around the wax. The silicate layer being connected, on the one hand, to the ceramic core and, on the other hand, to a further ceramic envelope in such a way that no deformations occur during the pouring of metal. The wall thicknesses which can be achieved with the method are between 0.25 mm and 1 mm for random solidification, and in the range between 0.076 mm and 1 mm for directionally solidified and single-crystal structures. The preferred field of application of the method is the production of single-crystal structures, for example for wings of orbital gliders, or gas-turbine guide blades as deflecting nozzles for aircraft engines. The method serves to raise the temperature stability of the hollow bodies cast in this way up to 2300.degree. C.
A method for the thermally controlled solidification of large castings having regions of thin wall structure is described in the article titled "A Thermal Analysis From Thermally Controlled Solidification (TCS) Trials On Investment Castings" by Patrick D. Ferro, Sanjay B. Shendye in "Superalloys", 1996, pages 531 to 535, The Minerals, Metals and Materials Society 1996. A casting produced according to this method differs from a directionally solidified casting or a single-crystal casting in particular by the grain size. Directionally solidified and single-crystal castings are distinguished by large and average grain sizes; in contrast, a casting produced according to the thermally controlled solidification method has an average grain size like a conventionally produced casting. In addition, a casting produced according to the thermally controlled solidification method has a consistent and uniform grain size in all the casting regions. A ratio of the temperature gradient G to the solidification speed R that leads to a microstructure having relatively small, equiaxed grains and minimal shrinkage is used in the thermally controlled solidification method. The method is carried out in a vacuum furnace in which a casting mold is heated via an induction heating system in a heating zone and is moved out of this heating zone for the solidification of the molten metal, so that cooling and solidification of the molten metal are effected by radiation cooling. The production of a casting mold and the construction of a corresponding furnace are described, for example, in U.S. Pat. No. 4,724,891. Described in this document is the production of a turbine-plant casing part, regions of which have a thin wall structure having an area of over 30 cm.sup.2 and a wall thickness of less than 0.125 cm. The ratio of the area of the region having a small wall thickness to the wall thickness is around at least 40.